1. Field of the Invention
The present invention relates to an image heating apparatus and a heater therefor, adapted for use in an image forming apparatus as represented by a copying machine, a printer or a facsimile apparatus, employing an image forming process such as an electrophotographic process or an electrostatic recording process, for the purpose of heating an image, formed by a transfer method or a direct method in an image forming process unit and supported by a recording material.
2. Related Background Art
As a heat fixing device in the image forming apparatus, there has conventionally been employed devices for a heat roller method or a film heating method. In particular, for the purpose of dispensing with the electric power supply to the heat fixing device in the stand-by state thereby minimizing the electric power consumption, there has been proposed a heat fixing device of a film heating method, in which a film is provided between the heater and the pressing roller for fixing the toner image on the recording material, as proposed for example in the Japanese Patent Laid-Open Application Nos. 63-313182, 2-157878, 4-44075 and 4-204980.
FIG. 8 schematically shows the principal parts of such a heat fixing device, which is provided with a heating member (hereinafter called heater) 41 fixed to and supported by a stay holder (support member) 42, and an elastic pressure roller 50 maintained in pressure contact, across a heat-resistant thin film (hereinafter called fixing film) 43, with the heater 41, forming therebetween a nip portion (fixing nip portion) N of a predetermined nip width.
The heater 41 is heated to and maintained at a predetermined temperature by the electric power supply. The fixing film 43 is composed of a cylindrical or endless belt-shaped member or a roll-fed web-shaped member which is conveyed in a direction indicated by an arrow a, by the rotating force of the pressure roller 50 or unrepresented drive means, in close contact with and sliding on the surface of the heater 41 at the fixing nip portion N.
In a state in which the heater 41 is heated to and maintained at the predetermined temperature and the fixing film 43 is conveyed in the direction a, if a recording material P bearing thereon an unfixed toner image t, which is to be heated, is inserted between the fixing film 43 at the fixing nip portion N and the pressure roller 50, the recording material P is closely contacted with a face of the fixing film 43 and introduced into the fixing nip portion N together with the fixing film 43. In the fixing nip portion N, the toner image t is heated by the heater 41 through the fixing film 43 whereby the toner image t is heat fixed to the recording material P. After passing the fixing nip portion N, the recording material is separated from the fixing film 43 and conveyed further.
The heater 41 constituting the heating member is generally composed of a ceramic heater, which is formed by providing a face (opposed to the fixing film 43) of an electrically insulating ceramic substrate 41a with a good thermal conductivity and a low heat capacity, composed for example of alumina, with an electrothermal heat-generating resistor layer 41b composed for example of silver-palladium (Ag/Pd), Ta.sub.2 N etc. and extended in the longitudinal direction of the substrate (perpendicular to the plane of the drawing) for example by screen printing, and coating the surface bearing such heat-generating resistor layer with a thin protective glass layer 41c.
In this ceramic heater 41, the heat-generating resistor layer 41b generates heat upon receiving an electric power supply, whereby the temperature of the entire heater including the ceramic substrate 41a and the protective glass layer 41c is elevated rapidly. The temperature rise in the heater 41 is detected by a temperature detecting sensor 44 provided at the back of the heater and is fed back to an unrepresented power supply control unit, which controls the electric power supply to the heat-generating resistor layer 41b in such a manner that the heater temperature detected by the temperature detector 44 is maintained at a substantially constant temperature (fixing temperature). In this manner the heater 41 is heated to and maintained at the predetermined fixing temperature.
The fixing film 43 is made as thin as 20 to 70 .mu.m in order that the heat of the heater 41 can be efficiently transferred to the recording material P at the fixing nip portion N. The fixing film 43 has a three-layered structure, consisting of a film base layer, a primer layer and a releasing layer, with the film base layer facing the heater 41 and the releasing layer facing the pressure roller 50. The film base layer is composed for example of polyimide, polyamidimide or PEEK which has higher insulating property than the protective glass layer 41c and which is also provided with a high heat resistance and a high elasticity. The film base layer maintains the mechanical strength, such as tear strength, of the entire fixing film 43. The primer layer is made as thin as 2 to 6 .mu.m. The releasing layer functions to prevent the toner offsetting to the fixing film 43 and is composed of a coating of fluorinated resin such as PFA, PTFE or FEP with a thickness of about 10 .mu.m.
The stay holder 42, composed for example of a heat-resistant plastic member, supports the heater 41 and functions also as a conveying guide member for the fixing film 43.
In the heat fixing device utilizing film heating method with such thin fixing film 43, because of the high rigidity of the ceramic heater 41, the pressure roller 50 having the elastic layer 51 becomes flat, following the shape of the flat lower face of the ceramic heater 41, thereby forming a nip portion N of a predetermined width at the contact position, and a quick-start heat fixing is achieved by heating such fixing nip portion N only.
FIG. 9 shows the details of the structure of the heater 41 employed in the heat fixing device of the above-explained film fixing method. The width W of the heat-generating resistor layer 41b of the heater 41 is contained within the fixing nip portion N, for fixing the toner image borne on the recording material through the fixing film 43. Consequently the heat generated by the heat-generating resistor layer 41b of the heater 41, by the electric power supply thereto, is given to the recording material P conveyed between the fixing film 43 and the pressure roller 50, thereby fusing and fixing the toner image t on the recording material P.
Behind the heater 41, as shown in FIG. 9, there are provided a temperature detector 44 such as a thermistor and a thermal protector 45 composed for example of a temperature fuse or a thermoswitch for shutting down the power supply to the heat-generating resistor layer 41b of the heater 41 in case of an abnormal operating condition, and these members are positioned within the conveying range of the recording material P of minimum width, that can be conveyed on the image forming apparatus.
The temperature detector 44 is positioned at the back of the heater 41, at the approximate center of the width W of the heat-generating resistor layer 41b, in order that the heat therefrom can be easily received. Also the thermo protector 45 is positioned at the back of the heater 41, at the approximate center of the width W of the heat-generating resistor layer 41b.
In the heat fixing device of the above-explained film heating method, however, the heat capacity of the pressure roller 50, the stay holder 42, the heater 41 etc. is made as small as possible, in order to ensure quick-starting performance. In such a situation, the position of the temperature detector 44 at the back of the heater 41, in addition to the performance thereof, has significant influence on the fixing of the toner image on the recording material and on the offsetting thereof, and has therefore to be determined precisely.
FIG. 10 is a chart showing the temperature distribution on the surface of the heater 41, from the upstream side of the fixing nip N to the downstream side thereof, while the fixing film is conveyed, wherein the abscissa indicates the position in the fixing nip N while the ordinate indicates the surface temperature of the heater. As will be understood from this chart, the surface temperature of the heater 41 is highest an area close to the center of the heat-generating resistor layer 41b, but becomes lower toward the upstream or downstream side in the fixing nip N. The temperature is higher in the downstream side than in the upstream side, because the upstream side receives the unheated fixing film while the sufficiently heated fixing film is discharged toward the downstream side.
In case the position of the temperature detector 44, provided in contact with the rear face of the heater 41, is displaced in the upstream or downstream direction, the temperature detected by the temperature detector 44 becomes different from the actual heating state of the heater in individual unit of the heat fixing device. For this reason, each heat fixing device has a higher possibility of causing defects such as insufficient fixing or toner offset by a high temperature. In order to avoid such drawbacks, the position of the temperature detector 44, on the rear face of the heater 41, has to be defined more precisely with a smaller tolerance of positioning, whereby the productivity of the heater 41 is inevitably lowered.
In addition, the thermo protector 45 such as a temperature fuse is provided for shutting down the power supply to the heat-generating resistor layer 41b in case of an abnormal heating state in such power supply, caused by a failure in the control unit for controlling the power supply to the heat-generating resistor layer 41b or in a safety circuit.
In case the thermo protector 45 is provided in contact with the rear face of the heater 41 corresponding to the central part of the heat-generating resistor layer 41b, it is therefore in an area of the highest temperature also in the normal state of use, so that the functioning temperature of such thermo protector has to be set higher than the highest temperature reached in the ordinary state of use. For this reason it has been difficult to improve the response speed of the thermo protector 45 for shutting down the power supply to the heat-generating resistor layer 41b in case of an abnormal heating state.
Also the thermistor constituting the temperature detector and the thermo protector are mounted with an adhesive material to the heater board, and such adhesive material has been associated with a drawback of being thermally deteriorated after prolonged exposure to heat, thus losing the adhesive power and thereby rendering the temperature detection unstable.